With modern methods of materials processing, structures can now be fabricated on the nanometer-length scale. As is well-known in the art, techniques developed in the semiconductor industry (e.g. electron beam lithography) can define complicated patterns with nanometer resolution. However, since these techniques are typically restricted to working at a material interface or surface layer (that is, in a typically two-dimensional format), much effort is required to use these methods to define a pattern in three dimensions. In particular, many layers of such a two-dimensional patterned material must typically be united to create a three-dimensionally patterned material. Many steps are required to produce each layer, and it therefore becomes prohibitive, both in terms of cost and time, to use these techniques to build multi-layered structures. Accordingly, a need exists for a simple method to make materials which are patterned in three dimensions. The present invention describes how to utilize recent developments in the chemistry of colloidal nanometer-scale particles (nanocrystals) to solve this problem.
It is presently known how to synthesize nanocrystals of a large variety of materials, including semiconductors, metals, and insulators, which are extremely homogeneous in terms of their size, shape, structure, and composition. In addition, it has been shown that under the proper conditions, the nanocrystals form close-packed solids, in which the nanocrystals are in contact but have not fused. See C. Murray et al., "Self-Organization of CdSe Nanocrystallites into Three-Dimensional Quantum Dot Superlattices," Science, Vol. 270, pp. 1335-1338 (Nov. 24, 1995). Close-packed nanocrystals, referred to herein as "quantum-dot solids," are artificial materials in which both the properties of the individual nanocrystalline building blocks and the interaction between them can be controlled. Therefore, the behavior of the solid can be tailored to fit a specific need.
It is believed that no method has yet been described by which a quantum-dot solid can be patterned in three dimensions. Such a method would be useful for construction of complicated optoelectronic devices which take advantage of the properties of these materials. For example, a single large-scale device could contain many quantum-dot solid "elements," such as photodiodes, light-emitting diodes, lasers, optical switches, and the like, all of which could be patterned on a single three-dimensional "chip." In addition, quantum-dot solids which are ordered in three dimensions are useful as "photonic" materials, which are discussed further below.
Prior art thin films and corresponding methods for making them, such as those disclosed in U.S. Pat. No. 5,262,357 to Alivisatos et al., U.S. Pat. No. 5,491,114 to Goldstein, U.S. Pat. No. 5,576,248 to Goldstein, and U.S. Pat. No. 5,711,803 to Pehnt et al., have demonstrated that when a thin layer of nanocrystals is deposited on an interface, the nanocrystals can be fused by heat to form a solid film. Significantly, these methods show that the nanocrystals fuse under temperatures much lower than the bulk melting temperature. Furthermore, U.S. Pat. No. 5,559,057 to Goldstein discloses a process by which such thin films can be patterned in two-dimensions. However, this process is again limited to an interface region, and extension of this method to patterns in three dimensions is difficult as described above.
In contrast to these prior art methods, in one embodiment of the present invention, a quantum-dot solid which is patterned in three dimensions can be obtained. In another embodiment, this material can be processed further. Specifically, by annealing or sintering this three-dimensionally patterned quantum-dot solid, the present invention also provides three-dimensional conventional solid structures in a straightforward manner.
One particular application in which a method for producing materials with a three-dimensional pattern is useful is in photonic crystals. A review of the properties and applications of such materials can be found in an article by Joannopoulos et al. entitled "Photonic Crystals: Putting a New Twist on Light," Nature, Vol. 386, pp. 143-149 (Mar. 13, 1997). Simply stated, a photonic crystal is a material with a periodic index of refraction. When the modulation of the index occurs on a length scale comparable to the wavelength of light, the material can modify the propagation of the photon through the material via diffraction. The extreme example is a photonic crystal which possesses a complete photonic band gap, a range of energies for which the photon cannot propagate in any direction inside the material.
Producing a photonic crystal is difficult, however, because one must fabricate a structure which is patterned and highly ordered in three dimensions. In addition, one must be able to pattern materials having a high index of refraction, such as semiconductors. Traditional semiconductor processing techniques (e.g. electron beam lithography) experience difficulty defining such patterns as described above.
Milstein et al. have described general methods for preparing photonic band gap materials in which the pores of a reticulated template are filled with a high index material. See U.S. Pat. Nos. 5,385,114, 5,651,818 and 5,688,318. The high index material is incorporated into the template either as a liquid or gas and then solidified. The template may then be removed by chemical means. Furthermore, Imhof et al. have described a method in which the template is filled by a gel. See Imhof et al., "Ordered Macroporous Materials by Emulsion Templating," Nature, Vol. 389, pp. 948-951 (Oct. 30, 1997).
In contrast to this prior art, the present invention, insofar as it pertains to photonic band gap materials, is an improvement over the prior art in that it allows nanometer-scale particles to fill the template. Moreover, unlike the Milstein et al. liquid-filling method, the present invention does not require the extreme temperatures typically needed for melting high index materials. Further, unlike the Milstein et al. gas-filling method, the present invention does not require a deposition chamber, which is expensive and limits the total sample thickness attainable. The present invention is simpler, does not require a complicated apparatus, and is more flexible, both in terms of selecting the fill-material and the template. Unlike the method of Imhof et al., the present invention is not limited to metal oxides (such as alumina, silica, titania, zirconia, etc.) as the fill-material. Any material which can be synthesized as a nanometer-sized particle and suspended as a colloid can be utilized as the fill-material in the present invention.